Integrated Circuit Devices with Well Regions

ABSTRACT

A method includes forming a deep well region of a first conductivity type in a substrate, implanting a portion of the deep well region to form a first gate, and implanting the deep well region to form a well region. The well region and the first gate are of a second conductivity type opposite the first conductivity type. An implantation is performed to form a channel region of the first conductivity type over the first gate. A portion of the deep well region overlying the channel region is implanted to form a second gate of the second conductivity type. A source/drain implantation is performed to form a source region and a drain region of the first conductivity type on opposite sides of the second gate. The source and drain regions are connected to the channel region, and overlap the channel region and the first gate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and claims priority to U.S. patent application Ser. No. 13/539,027, filed on Jun. 29, 2012, and entitled, “Integrated Circuit Devices with Well Regions and Methods for Forming the Same,” which application is hereby incorporated herein by reference.

BACKGROUND

Integrated circuits include Metal-Oxide-Semiconductor (MOS) devices, diodes, capacitors, varactors, resistors, and the like. The formation processes of these devices need to be integrated, so that they can be formed in the same semiconductor chips.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIGS. 1 through 10 are cross-sectional views of intermediate stages in the manufacturing of an integrated circuit structure in accordance with some exemplary embodiments;

FIGS. 11 through 15 illustrate the cross-sectional views of various devices in accordance with exemplary embodiments; and

FIGS. 16 through 23 illustrate various layouts in accordance with exemplary embodiments.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussed in detail below. It should be appreciated, however, that the embodiments provide many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are illustrative, and do not limit the scope of the disclosure.

An integrated circuit structure that may be used for forming varying devices and the method of forming the same are provided in accordance with exemplary embodiments. The intermediate stages of forming the integrated circuit structure are illustrated. The variations of the embodiments are discussed. Throughout various views and illustrative embodiments, like reference numbers are used to designate like elements. Although the illustrated exemplary embodiments provide the methods of forming the integrated circuit structure having p-type bottom and top gates and an n-type channel, one of ordinary skill in the art will realize that the teaching is readily available for the formation of n-type bottom and top gates and a p-type channel, with the conductivity types of the respective doped semiconductor regions inverted.

FIG. 1 illustrates the formation of isolation regions 22 at the top surface of substrate 20, which may be a part of wafer 10. Substrate 20 is a semiconductor substrate, which may be a silicon substrate, although other semiconductor materials such as germanium, silicon germanium, III-V compound semiconductor materials, or the like, may be used. Isolation regions 22 may be Shallow Trench Isolation (STI) regions, and hence are referred to STI regions 22 throughout the description, although isolation regions 22 may also be of other types such as field oxides. Deep n-well region 24 is formed in substrate 20. In some embodiments, the formation of deep n-well region 24 includes forming photo resist 26, and implanting an n-type impurity such as phosphorous, indium, or the like into substrate 20. Photo resist 26 is then removed. The bottom surface of deep n-well region 24 is lower than the bottom surface of STI regions 22. An exemplary impurity concentration in deep n-well region 24 is between about 1E13/cm³ and about 1E15/cm³.

Referring to FIG. 2, buried well region 28, which may be of p-type, is formed. Throughout the description, buried well region 28 is alternatively referred to as bottom gate 28 since it may act as a bottom gate of the resulting Field-Effect Transistor (FET) device in some exemplary embodiments. The formation includes forming and patterning photo resist 30, and implanting a p-type impurity into an intermediate level of deep n-well region 24. Photo resist 30 is then removed. In some embodiments, bottom gate 28 is spaced apart from the bottoms of STI regions 22 by distance Di, which may be between about 0.1 μm and about 1 μm, for example. It is appreciated, however, that the values recited throughout the description are merely examples, and may be changed to different values.

Next, as shown in FIG. 3, p-well regions 32 are formed through an implantation step, wherein photo resist 34 is used as an implantation mask. The energy of the implantation is controlled so that p-well regions 32 contact bottom gate 28. P-well regions 32 may extend from the top of substrate 20 into deep n-well region 24. Although p-well regions 32 are referred to in a plural form, when viewed in a top view of the structure in FIG. 3, the illustrated p-well regions 32 may be portions of a same continuous p-well region 32, which may form a full ring in the top view. In some embodiments, the ring of p-well region 32 and bottom gate 28 in combination isolate the n-type regions inside the ring from other portions of substrate 20 that are outside the ring and below bottom gate 28. P-well region 32 has a p-type impurity concentration similar to that of bottom gate 28 in some embodiments, although the p-type impurity concentration may also be greater than or less than that of bottom gate 28.

FIG. 4 illustrates the formation of p-type pickup regions 36, which may again be formed through an implantation step, wherein photo resist 35 is used as an implantation mask. P-type pickup regions 36 may also form a ring in the top view of the structure shown in FIG. 4. In some exemplary embodiments, p-type pickup regions 36 have a p-type impurity concentration between about 10¹⁹/cm³ and about 10²¹/cm³.

FIG. 5 illustrates the formation of channel region 42, which may be formed through implantation, wherein photo resist 40 may be used as an implantation mask. The formation of channel region 42 may be performed using a same lithography mask as for forming bottom gate 28. Accordingly, channel region 42 may overlap and be aligned to bottom gate 28. In the implantation step, additional n-type impurities are implanted to the region overlying bottom gate 28, and hence the impurity concentration of channel region 42 is adjusted to a desirable level, for example, between about 1E13/cm³ and about 1E14/cm³. Channel region 42 and the underlying bottom gate 28 form a p-n junction.

Referring to FIG. 6, n-type source/drain enhancement regions 48 (also referred to as source/drain regions) are formed through an implantation step, wherein photo resist 46 is formed as an implantation mask. N-type source/drain enhancement regions 48 may have an n-type impurity concentration between about 1E13/cm³ and about 1E15/cm³. In the meantime, deep n-well pickup regions 49 are also formed.

Next, as shown in FIG. 7, some components of N-type Metal-Oxide-Semiconductor (NMOS) device 100 and P-type Metal-Oxide-Semiconductor (PMOS) device 200 are formed. NMOS device 100 includes gate stack 110, which includes gate dielectric 102, gate electrode 104, hard mask layer 106, and gate spacers 108. Lightly Doped Drain/source (LDD) regions 112, which are of n-type, are also formed in substrate 20, for example, through an implantation. Halo/pocket regions 114, which are of p-type, may also be formed. PMOS device 200 includes gate stack 210, which includes gate dielectric 202, gate electrode 204, hard mask layer 206, and gate spacers 208. Lightly Doped Drain/source (LDD) regions 212, which are of p-type, are also formed in substrate 20, for example, through an implantation. Halo/pocket region 214, which are of n-type, may also be formed. In addition, well region 215 may be formed for PMOS device 200. The formation of gate dielectrics 102/202, gate electrodes 104/204, and hard mask layers 106/206 may include blanket depositing a gate dielectric layer, a gate electrode layer, and a hard mask layer, and then patterning the blanket layers.

In accordance with embodiments, the formation of n-type source/drain enhancement regions 48 (FIG. 6) and/or channel region 42 (FIG. 5) are performed before the formation of the MOS device components as in FIG. 7. Since the formation of MOS devices 100 and 200 includes a plurality of thermal processes such as anneals, the impurities in the n-type source/drain enhancement regions 48 and channel region 42 diffuse out during the thermal processes, and hence the impurities in n-type source/drain enhancement regions 48 may diffuse to the underlying regions. This may improve the connection between source/drain enhancement regions 48 and channel region 42, and result in a reduction in the respective resistance.

FIG. 8 illustrates the formation of source/drain regions 216 for PMOS device 200. Photo resist 52 is formed to mask the regions that are not to be implanted. During the formation of source/drain regions 216, p-type pickup regions 36 may be implanted again simultaneously to increase their impurity concentration. In alternative embodiments, p-type pickup regions 36 are not implanted when source/drain regions 216 are formed. Furthermore, a region overlying channel region 42 is implanted to form top gate 53, which is of p-type. Top gate 53 is in contact with channel region 42, and forms a p-n junction with channel region 42.

FIG. 9 illustrates the formation of source/drain regions 116 for NMOS device 100. Photo resist 54 is formed to mask the regions that are not to be implanted. During the formation of source/drain regions 116, source/drain enhancement regions 48 may be implanted again simultaneously to increase their impurity concentration. In alternative embodiments, source/drain enhancement regions 48 are not implanted when source/drain regions 116 are formed. In the resulting structure, the impurity concentration of source/drain enhancement regions 48 is further increased. Source/drain enhancement regions 48 are referred to hereinafter as source/drain regions 56, which include source region 56A and drain region 56B.

Next, as shown in FIG. 10, Resistive Protection Oxide (RPO) regions 58 are formed to cover the interfaces between doped regions 36, 49, 53, 56 and their respective neighboring STI regions 22. RPO regions 58 may protect the interface regions from the undesirable etch caused in the subsequent formation of contact plugs (not shown), and may help reduce the leakage currents occurring in the interface regions. In subsequent process steps, hard masks layers 106 and 206 (marked in FIG. 9) are removed. Additional regions such as silicide regions, contact plugs, Inter-Layer Dielectric (ILD), metal layers (note shown), and the like, are formed.

As shown in FIG. 10, source/drain regions 56 include source region 56A and drain region 56B, which have the same conductivity type as, and are electrically coupled to, channel region 42. Source region 56A and drain region 56B are further spaced apart from each other by top gate 53, which is of an opposite conductivity type than source/drain regions 56. Bottom gate 28 may be accessed through p-well pickup regions 36 and p-well regions 32. For example, a voltage may be applied to bottom gate 28 by applying the voltage to p-well pickup regions 36.

The structure shown in FIG. 10 may be used to form various integrated circuit devices such as Junction FET (JFET) devices, varactors, resistors, Bipolar Junction Transistors (BJTs), diodes, or the like. Since the formation processes of these devices share the same process steps, the manufacturing cost is reduced. FIGS. 11 through 15 illustrate some exemplary devices formed from the structure shown in FIG. 10. The formation of the devices in FIGS. 11 through 15 may all start from the structure shown in FIG. 10, except that the electrical connections, which may include contact plugs, metal lines, vias, and the like are different, so that the resulting devices are different, as shown in FIGS. 11 through 15. The electrical connections are symbolized by lines in FIGS. 11 through 15.

Referring to FIG. 11, JFET 220 is formed, and includes source region 56A, drain region 56B, top gate 53, and bottom gate 28. Channel region 42 is controlled by top gate 53 and bottom gate 28. By adjusting voltages VG1 and VG2 that are applied on top gate 53 and bottom gate 28, respectively, the channel in channel region 42 may be turned on or pinched off by the depletion region formed between top gate and channel region 42, and the depletion region formed between bottom gate 28 and channel region 42. The current I flowing through the channel is illustrated. Although not shown, a voltage source may be connected to top gate 53 and bottom gate 28 in order to provide voltages VG1 and VG2.

Referring to FIG. 12, varactors 302, 304, and 306 are formed. For example, source region 56A and drain region 56B may be shorted. Accordingly, varactor 302 is formed between top gate 53 and channel region 42, wherein source/drain regions 56 and channel region 42 are used as a capacitor electrode, and top gate 53 acts as another capacitor electrode. Varactors 304 are formed between bottom gate 28/p well regions 32 and channel region 42, wherein source/drain regions 56 and channel region 42 are used as a capacitor electrode, and bottom gate 28 or p well regions 32 forms another capacitor electrode. Furthermore, bottom gate 28/p well regions 32 may also form varactors 306 with deep n-well region 24, wherein pickup regions 49 are used to access varactors 306.

In FIG. 13, resistors 402 and/or 404 may be formed. For example, channel region 42 forms resistor 402, which is coupled between source region 56A and drain region 56B. Furthermore, bottom gate 28 may form resistor 404, which is coupled between two separated p-well pickup regions 36. In these embodiments, however, p-well regions 32 that are connected to opposite ends of resistor 404 are separate p-well regions rather than a p-well ring. Furthermore, p-well pickup regions 36 that are connected to opposite ends of resistor 404 are also separate p-well regions rather than a p-well ring.

In FIG. 14, diodes 502, 504, and 506 are formed. Diode 502 is formed between top gate 53 and channel region 42, wherein source/drain regions 56 may be used as the cathode pickup. Diode 504 is formed between bottom gate 28 and channel region 42, wherein source/drain regions 56 are used as the cathode pickup, and p-well pickup regions 36 are used as the anode pickup. Diode 506 is formed between bottom gate 28 and deep n-well region 24, wherein p-well pickup regions 36 may be used as the anode pickup, and deep n-well pickup regions 49 may be used as the cathode pickup.

In FIG. 15, PNP BJT 602 is formed, wherein channel region 42 is the base, and source/drain regions 56 are base pickup regions. One of top gate 53 and bottom gate 28 is the emitter of PNP BJT 602, and the other is the collector. NPN BJT 604 is also formed, wherein p-well regions 32 and bottom gate 28 are the base. Source/drain regions 56 act as one of the emitter and collector, and deep n-well region 24 act as the other one of the emitter and collector.

FIGS. 16 through 23 are exemplary layouts of the integrate circuit structures shown in FIGS. 10 through 15. It is appreciated that there are more varieties as how the integrate circuit structures may be laid out, and these varieties are also in the scope of various embodiments. In FIGS. 16 through 23, channel region 42 and bottom gate 28 are marked using dashed lines.

Referring to FIGS. 16 and 17, asymmetric layouts are illustrated, with the spacing S1 between drain region 56B and top gate 53 different from spacing S2 between source region 56A and top gate 53. Top gate 53 is disconnected from p-well pickup region 36, and hence is disconnected from bottom gate 28, although top gate 53 may also be connected to p-well pickup region 36 in alternative embodiments, similar to FIGS. 18 and 19. In FIG. 16, one top gate 53, one source region 56A, and one drain region 56B are formed. In FIG. 17, two top gates 53, two source regions 56A, and one drain region 56B are formed.

Referring to FIGS. 18 and 19, symmetric layouts are illustrated, with the spacing S1 between drain region 56B and top gate 53 substantially equal to spacing S2 between source region 56A and top gate 53. Top gate 53 is connected to p-well pickup region 36 in these embodiments, and hence is connected to bottom gate 28, although top gate 53 may also be disconnected from p-well pickup region 36 in alternative embodiments, similar to FIGS. 16 and 17. In FIG. 18, one top gate 53, one source region 56A, and one drain region 56B are formed. In FIG. 19, two top gates 53, two source regions 56A, and one drain region 56B are formed.

In FIGS. 20 through 23, each of top gate 53, source region 56A, and p-well pickup region 36 forms a ring encircling drain region 56B, with the outer rings encircling the inner rings. FIG. 20 illustrates a symmetric structure, with the spacing S1 between drain region 56B and top gate 53 substantially equal to spacing S2 between source region 56A and top gate 53. FIG. 21 illustrates an asymmetric structure, with the spacing S1 between drain region 56B and top gate 53 different from spacing S2 between source region 56A and top gate 53. FIG. 22 illustrates a plurality of drain regions 56B that are spaced apart from each other, for example, by STI regions 22. The plurality of drain regions 56B may be interconnected as a single drain region, or disconnected from each other in order to be connected to different voltages and nodes. FIG. 23 illustrates a dual gate structure, wherein there are two top gates 53A and 53B, which may be applied with different voltages and/or connected to different nodes. Both top gates 53A and 53B may affect the operation of the respective integrated circuit structure.

In accordance with embodiments, a method includes forming a deep well region of a first conductivity type in a substrate, implanting a portion of the deep well region to form a first gate, and implanting the deep well region to form a well region. The well region and the first gate are of a second conductivity type opposite the first conductivity type. The deep well region includes a portion connected to an end of the first gate. An implantation is performed to form a channel region over the first gate, wherein the channel region is of the first conductivity type. A portion of the deep well region overlying the channel region is implanted to form a second gate of the second conductivity type. A source/drain implantation is performed to form a source region and a drain region on opposite sides of the second gate. The source and drain regions are of the first conductivity type, and are connected to the channel region. The source region and the drain region overlap the channel region and the first gate.

In accordance with other embodiments, a method includes forming a deep well region of a first conductivity type in a substrate, implanting an intermediate portion of the deep well region to form a bottom gate, and implanting the deep well region to form a well ring extending from a top surface of the substrate to the bottom gate. The well ring and the bottom gate are of a second conductivity type opposite the first conductivity type. The well ring encircles the bottom gate. A portion of the deep well region overlying and contacting the bottom gate is implanted to form a channel region of the first conductivity type. A portion of the deep well region overlying the channel region is implanted to form a top gate of the second conductivity type. A source/drain implantation is performed to form a first source region and a first drain region on opposite sides of the top gate, wherein the first source and drain regions are of the first conductivity type, and are connected to the channel region. The first source region and the first drain region overlap the channel region and the bottom gate. After the first source/drain implantation, a gate stack of a MOS device is formed over the substrate. After forming the gate stack, a second source/drain implantation is performed to form second source and drain regions for the MOS device.

In accordance with yet other embodiments, a device includes a deep well region of a first conductivity type in a substrate, a bottom gate in the deep well region, and a well region extending from a top surface of the substrate to the bottom gate. The well region and the bottom gate are of a second conductivity type opposite the first conductivity type. The well region encircles the bottom gate in a plan view. A channel region of the first conductivity is disposed overlying and contacting the bottom gate. A top gate of the second conductivity type is disposed overlying the channel region and forming a p-n junction with the channel region. A source region and a drain region are on opposite sides of the top gate. The source and drain regions are of the first conductivity type, and are connected to the channel region. The source region and the drain region overlap the channel region and the bottom gate.

In accordance with yet other embodiments, a device includes a substrate including a semiconductor material, a plurality of shallow trench isolation (STI) regions at a top surface of the substrate, a first well region of a first conductivity type embedded in the substrate, and a bottom gate in the first well region. The bottom gate is of a second conductivity type opposite the first conductivity type. A second well region extends from the top surface of the substrate to the bottom gate. The second well region encircles the bottom gate in a plan view. The second well region is of a second conductivity type opposite the first conductivity type. A channel region of the first conductivity type overlies and contacts the bottom gate. A first top gate and a second top gate of the second conductivity type overlie the channel region. The first top gate and the second top gate form p-n junctions with the channel region. A first source region and a second source region are disposed on opposite sides of the first top gate and the second top gate so that a straight line drawn from the first source region to the second source region would intersect the first top gate and the second top gate. The first and second source regions are of the first conductivity type. The first and second source regions are connected to the channel region. The first and second source regions overlap the channel region and the bottom gate. A drain region is disposed between the first top gate and the second top gate. The drain region is of the first conductivity type. The drain region is connected to the channel region. The drain region overlaps the channel region and the bottom gate.

In accordance with yet other embodiments, a semiconductor device includes a first well region of a first conductivity type in a substrate, a bottom gate in the first well region, and a channel region of the first conductivity type overlying and contacting the bottom gate. At least one drain region of the first conductivity type is connected to the channel region. The at least one drain region overlaps the channel region and the bottom gate. A top gate overlies the channel region and forms a p-n junction with the channel region. The top gate is of a second conductivity type opposite the first conductivity type. The top gate encircles the at least one drain region in a plan view. A source region is connected to the channel region. The source region is of the first conductivity type. The source region overlaps the channel region and the bottom gate. The source region encircles the top gate in a plan view. A second well region extends from a top surface of the substrate to the bottom gate. The second well region is of the second conductivity type. The second well region encircles the source region in a plan view.

Although the embodiments and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the embodiments as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. In addition, each claim constitutes a separate embodiment, and the combination of various claims and embodiments are within the scope of the disclosure. 

What is claimed is:
 1. A device, comprising: a deep well region of a first conductivity type in a substrate; a bottom gate in the deep well region; a well region extending from a top surface of the substrate to the bottom gate, wherein the well region and the bottom gate are of a second conductivity type opposite the first conductivity type, and wherein the well region encircles the bottom gate in a plan view; a channel region of the first conductivity type overlying and contacting the bottom gate; a top gate of the second conductivity type overlying the channel region and forming a p-n junction with the channel region; and a source region and a drain region on opposite sides of the top gate, wherein the source and drain regions are of the first conductivity type, and are connected to the channel region, and wherein the source region and the drain region overlap the channel region and the bottom gate.
 2. The device of claim 1, wherein the well region further encircles the channel region, the top gate, the source region, and the drain region.
 3. The device of claim 1, wherein the top gate, the well region, the source region, the drain region, and the bottom gate form a Junction Field Effect Transistor (JFET), and wherein the top gate and the bottom gate are configured to pinch off the channel region.
 4. The device of claim 1, further comprising electrical connections coupled to the top gate, the well region, the source region, the drain region, and the bottom gate to form a varactor, with the source region and drain region electrically interconnected to form a capacitor electrode of the varactor.
 5. The device of claim 1, further comprising electrical connections coupled to the top gate, the well region, the source region, the drain region, and the bottom gate to form a resistor, with the bottom gate or the channel region forming the resistor.
 6. The device of claim 1, further comprising electrical connections coupled to the top gate, the well region, the source region, the drain region, and the bottom gate to form a diode, with the channel region being a first one of a cathode and an anode of the diode, and the top gate or the bottom gate being a second one of the cathode and the anode.
 7. The device of claim 1, further comprising electrical connections coupled to the top gate, the well region, the source region, the drain region, and the bottom gate to form a bipolar junction transistor.
 8. The device of claim 1, wherein the top gate is connected to the well region.
 9. A device, comprising: a substrate comprising a semiconductor material; a plurality of shallow trench isolation (STI) regions at a top surface of the substrate; a first well region of a first conductivity type embedded in the substrate; a bottom gate in the first well region, wherein the bottom gate is of a second conductivity type opposite the first conductivity type; a second well region extending from the top surface of the substrate to the bottom gate, wherein the second well region encircles the bottom gate in a plan view, and wherein the second well region is of a second conductivity type opposite the first conductivity type; a channel region of the first conductivity type overlying and contacting the bottom gate; a first top gate and a second top gate, wherein the first top gate and the second top gate are of the second conductivity type overlying the channel region and wherein the first top gate and the second top gate form p-n junctions with the channel region; a first source region and a second source region disposed on opposite sides of the first top gate and the second top gate so that a straight line drawn from the first source region to the second source region would intersect the first top gate and the second top gate, wherein the first and second source regions are of the first conductivity type, wherein the first and second source regions are connected to the channel region, and wherein the first and second source regions overlap the channel region and the bottom gate; and a drain region disposed between the first top gate and the second top gate, wherein the drain region is of the first conductivity type, wherein the drain region is connected to the channel region, and wherein the drain region overlaps the channel region and the bottom gate.
 10. The device of claim 9, wherein a first spacing between the drain region and the first top gate is substantially different from a second spacing between the first source region and the first top gate.
 11. The device of claim 9, wherein a first spacing between the drain region and the second top gate is substantially equal to a second spacing between the second source region and the second top gate.
 12. The device of claim 9, wherein the first top gate and the second top gate are connected to the second well region.
 13. The device of claim 9, further comprising a Metal-Oxide-Semiconductor (MOS) device formed over the substrate.
 14. A semiconductor device, comprising: a first well region of a first conductivity type in a substrate; a bottom gate in the first well region; a channel region of the first conductivity type overlying and contacting the bottom gate; at least one drain region of the first conductivity type connected to the channel region, wherein the at least one drain region overlaps the channel region and the bottom gate; a top gate overlying the channel region and forming a p-n junction with the channel region, wherein the top gate is of a second conductivity type opposite the first conductivity type, and wherein the top gate encircles the at least one drain region in a plan view; a source region connected to the channel region, wherein the source region is of the first conductivity type, wherein the source region overlaps the channel region and the bottom gate, and wherein the source region encircles the top gate in a plan view; and a second well region extending from a top surface of the substrate to the bottom gate, wherein the second well region is of the second conductivity type, and wherein the second well region encircles the source region in a plan view.
 15. The device of claim 14, wherein a first spacing between the at least one drain region and the top gate is substantially equal to a second spacing between the source region and the top gate.
 16. The device of claim 14, wherein a first spacing between the at least one drain region and the top gate is substantially different from a second spacing between the source region and the top gate.
 17. The device of claim 14, further comprising another top gate, wherein the another top gate overlies the channel region and forms a p-n junction with the channel region, wherein the another top gate is of a second conductivity type opposite the first conductivity type, and wherein the another top gate encircles the top gate in a plan view.
 18. The device of claim 14, wherein the at least one drain region comprises a plurality of drain regions of the first conductivity type connected to the channel region, wherein the plurality of drain regions overlap the channel region and the bottom gate, and wherein the top gate encircles the plurality of drain regions in a plan view.
 19. The device of claim 18, wherein the plurality of drain regions are spaced apart from each other by shallow trench isolation (STI) regions.
 20. The device of claim 18, wherein the plurality of drain regions are interconnected as a single drain region. 